Close-up lens, imaging apparatus, and method for focusing close-up lens

ABSTRACT

Providing a close-up lens with superb optical performance capable of focusing from infinity to a close distance allowing a life-size magnification by an internal focusing system with small moving amounts of focusing lens groups, an imaging apparatus, and a method for focusing a close-up lens. The close-up lens includes four lens groups, and is able to focus an object with an imaging magnification from β=0 through at least β=−0.5, wherein upon focusing, a first lens group and a fourth lens group are fixed with respect to an image plane, and a second lens group and a third lens group are moved along an optical axis.

The disclosure of the following priority application is herein incorporated by reference:

Japanese Patent Application No. 2007-065400; and

Japanese Patent Application No. 2008-034053.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a close-up lens, and in particular to a close-up lens with an internal focusing system having a constant total lens length, an imaging apparatus, and a method for focusing a close-up lens.

2. Related Background Art

A photo-taking lens for a close-up photograph has been proposed in, for example, Japanese Patent Application Laid-Open No. 2005-004041. Unlike an ordinary photo-taking lens, a close-up lens has to focus an object locating from infinity to a close distance allowing a life-size magnification or in the vicinity thereof, so that moving amounts of lens groups upon focusing necessarily become large. Accordingly, it is unsuitable for auto focus. When you try to focus from infinity to a close distance providing a life-size magnification by extending the whole lens system in a body, you have to move it a distance same as the focal length thereof. On this occasion, since variations in spherical aberration and the image plane cannot be suppressed, a lot of lenses have adopted a floating system in which moving speed of a portion of lens group differs from that of the whole lens system.

However, even a close-up lens with a floating system has still had a large moving amount of the lens group, so that it is unsuited for auto focus.

SUMMARY OF THE INVENTION

The present invention is made in view of the aforementioned problems and has an object to provide a close-up lens with superb optical performance capable of focusing from infinity to a close distance allowing a life-size magnification by an internal focusing system with small moving amounts of focusing lens groups, an imaging apparatus, and a method for focusing a close-up lens.

According to a first aspect of the present invention, there is provided a close-up lens comprising four lens groups, and capable of focusing an object with an imaging magnification from β=0 through at least β=−0.5; wherein upon focusing, a first lens group and a fourth lens group are fixed with respect to an image plane, and a second lens group and a third lens group are moved along an optical axis.

In a close-up lens according to the first aspect of the present invention, it is preferable that at least one aspherical surface is included in the close-up lens.

In a close-up lens according to the first aspect of the present invention, it is preferable that the first lens group is composed of three lenses or less.

In a close-up lens according to the first aspect of the present invention, it is preferable that the fourth lens group is composed of at least three lenses or more.

In a close-up lens according to the first aspect of the present invention, it is preferable that the first lens group includes at least one aspherical surface.

In a close-up lens according to the first aspect of the present invention, it is preferable that the second lens group includes at least one aspherical surface.

According to a second aspect of the present invention, there is provided a close-up lens comprising four lens groups, and capable of focusing an object with an imaging magnification from β=0 through at least β=−1.0; wherein upon focusing, a first lens group and a fourth lens group are fixed with respect to an image plane, and a second lens group and a third lens group are moved along an optical axis; and the following conditional expressions being satisfied: 1.5<β0<2.3 0.3<β1<0.9 where β0 denotes a lateral magnification of the second lens group upon focusing on infinity, and β1 denotes a lateral magnification of the second lens group upon an imaging magnification β=−1.0.

In a close-up lens according to the second aspect of the present invention, it is preferable that at least one aspherical surface is included in the close-up lens.

In a close-up lens according to the second aspect of the present invention, it is preferable that the first lens group is composed of three lenses or less.

In a close-up lens according to the second aspect of the present invention, it is preferable that the fourth lens group is composed of at least three lenses or more.

In a close-up lens according to the second aspect of the present invention, it is preferable that the first lens group includes at least one aspherical surface.

In a close-up lens according to the second aspect of the present invention, it is preferable that the second lens group includes at least one aspherical surface.

According to a third aspect of the present invention, there is provided an imaging apparatus equipped with the close-up lens according to the first aspect.

According to a fourth aspect of the present invention, there is provided an imaging apparatus equipped with the close-up lens according to the second aspect.

According to a fifth aspect of the present invention, there is provided a method for focusing a close-up lens capable of focusing an object with an imaging magnification from β=0 through at least β=−0.5, the method comprising steps of: providing the close-up lens including four lens groups; fixing a first lens group and a fourth lens group with respect to an image plane upon focusing; and moving a second lens group and a third lens group along an optical axis upon focusing.

In a fifth aspect of the present invention, it is preferable to further include a step of: including at least one aspherical surface in the close-up lens.

According to a sixth aspect of the present invention, there is provided a method for focusing a close-up lens capable of focusing an object with an imaging magnification from β=0 through at least β=−1.0, the method comprising steps of: providing the close-up lens including four lens groups; satisfying the following conditional expressions; fixing a first lens group and a fourth lens group with respect to an image plane upon focusing; and moving a second lens group and a third lens group along an optical axis upon focusing: 1.5<β0<2.3 0.3<β1<0.9 where β0 denotes a lateral magnification of the second lens group upon focusing on infinity, and β1 denotes a lateral magnification of the second lens group upon the imaging magnification β=−1.0.

In a sixth aspect of the present invention, it is preferable to further include a step of: including at least one aspherical surface in the close-up lens.

The present invention makes it possible to provide an internal focusing close-up lens having an angle of view from about 37 degrees to 45 degrees, and an f-number of about 2.8 suitable for an optical instrument such as a film camera, an electronic still camera, and the like, capable of focusing by auto focus from infinity to a close distance allowing a life-size magnification with high optical performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 1 upon focusing on infinity.

FIGS. 2A, 2B, 2C are graphs showing various aberrations of the close-up lens according to Example 1, in which FIG. 2A is in a state focusing on infinity, FIG. 2B is in a state where an imaging magnification β=−0.5, and FIG. 2C is in a state where an imaging magnification β=−1.0.

FIG. 3 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 2 upon focusing on infinity.

FIGS. 4A, 4B, 4C are graphs showing various aberrations of the close-up lens according to Example 2, in which FIG. 4A is in a state focusing on infinity, FIG. 4B is in a state where an imaging magnification β=−0.5, and FIG. 4C is in a state where an imaging magnification β=−1.0.

FIG. 5 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 3 upon focusing on infinity.

FIGS. 6A, 6B, 6C are graphs showing various aberrations of the close-up lens according to Example 3, in which FIG. 6A is in a state focusing on infinity, FIG. 6B is in a state where an imaging magnification β=−0.5, and FIG. 6C is in a state where an imaging magnification β=−1.0.

FIG. 7 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 4 upon focusing on infinity.

FIGS. 8A, 8B, 8C are graphs showing various aberrations of the close-up lens according to Example 4, in which FIG. 8A is in a state focusing on infinity, FIG. 8B is in a state where an imaging magnification β=−0.5, and FIG. 8C is in a state where an imaging magnification β=−1.0.

FIG. 9 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 5 upon focusing on infinity.

FIGS. 10A, 10B, 10C are graphs showing various aberrations of the close-up lens according to Example 5, in which FIG. 10A is in a state focusing on infinity, FIG. 10B is in a state where an imaging magnification β=−0.5, and FIG. 10C is in a state where an imaging magnification β=−1.0.

FIG. 11 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 6 upon focusing on infinity.

FIGS. 12A, 12B, 12C are graphs showing various aberrations of the close-up lens according to Example 6, in which FIG. 12A is in a state focusing on infinity, FIG. 12B is in a state where an imaging magnification β=−0.5, and FIG. 12C is in a state where an imaging magnification β=−1.0.

FIG. 13 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 7 upon focusing on infinity.

FIGS. 14A, 14B, 14C are graphs showing various aberrations of the close-up lens according to Example 7, in which FIG. 14A is in a state focusing on infinity, FIG. 14B is in a state where an imaging magnification β=−0.5, and FIG. 14C is in a state where an imaging magnification β=−1.0.

FIG. 15 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 8 upon focusing on infinity.

FIGS. 16A, 16B, 16C are graphs showing various aberrations of the close-up lens according to Example 8, in which FIG. 16A is in a state focusing on infinity, FIG. 16B is in a state where an imaging magnification β=−0.5, and FIG. 16C is in a state where an imaging magnification β=−1.0.

FIG. 17 is a diagram showing an imaging apparatus equipped with a close-up lens according to Example 1.

DETAILED DESCRIPTION OF THE MOST PREFERRED EMBODIMENT

A close-up lens according to the present embodiment is going to be explained below.

A close-up lens according to the present embodiment includes four lens groups and is capable of focusing an object with an imaging magnification from β=0 through at least β=−0.5, wherein upon focusing, a first lens group and a fourth lens group are fixed with respect to an image plane, and a second lens group and a third lens group are moved along an optical axis. With this lens configuration, it becomes possible to take a close-up photograph with a large imaging magnification. In this case, the imaging magnification β=0 is a state upon focusing on infinity.

Moreover, a close-up lens according to the present embodiment includes four lens groups and is capable of focusing an object with an imaging magnification from β=0 through at least β=−1.0, wherein upon focusing, a first lens group and a fourth lens group are fixed with respect to an image plane, and a second lens group and a third lens group are moved along an optical axis, and the following conditional expressions (1) and (2) are satisfied: 1.5<β0<2.3  (1) 0.3<β1<0.9  (2) where β0 denotes a lateral magnification of the second lens group upon focusing on infinity, and β1 denotes a lateral magnification of the second lens group upon an imaging magnification β=−1.0.

Conditional expression (1) defines an appropriate range of the lateral magnification of the second lens group upon focusing on infinity. When the value β0 is equal to or exceeds the upper limit of conditional expression (1), the focal length of the second lens group becomes short, so that spherical aberration and curvature of field are both overcorrected.

On the other hand, when the value β0 is equal to or falls below the lower limit of conditional expression (1), the focal length of the second lens group becomes long, and both of spherical aberration and curvature of field become undercorrected, so that it is undesirable.

In order to secure the effect of the present embodiment, it is preferable to set the upper limit of conditional expression (1) to 2.10. In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (1) to 1.70.

Conditional expression (2) defines an appropriate range of a moving amount of the second lens group upon an imaging magnification β=−1.0. When the value β1 is equal to or exceeds the upper limit of conditional expression (2), a combined focal length of the first lens group and the second lens group upon the imaging magnification β=−1.0 becomes short, so that variation in various aberrations such as spherical aberration upon the imaging magnification from β=0 to β=−1.0 cannot be suppressed.

On the other hand, when the value β1 is equal to or falls below the lower limit of conditional expression (2), a combined focal length of the first lens group and the second lens group upon the imaging magnification β=−1.0 becomes long, both of spherical aberration and curvature of field become undercorrected, so that it is undesirable.

In order to secure the effect of the present embodiment, it is preferable to set the upper limit of conditional expression (2) to 0.8. In order to secure the effect of the present embodiment, it is preferable to set the lower limit of conditional expression (2) to 0.5.

A close-up lens according to the present embodiment preferably includes at least one aspherical surface.

With constructing in such a manner, it becomes possible to excellently correct various aberrations such as spherical aberration and coma.

In a close-up lens according to the present embodiment, the first lens group is preferably composed of three lenses or less.

With constructing in such a manner, it becomes possible to secure a wide moving space for the second lens group with correcting spherical aberration and coma, so that variation in various aberrations such as spherical aberration upon focusing can be suppressed.

In a close-up lens according to the present embodiment, the fourth lens group is preferably composed of at least three lenses or more.

With constructing in such a manner, it becomes possible to excellently correct various aberrations such as curvature of field.

In a close-up lens according to the present embodiment, it is preferable that the first lens group includes at least one aspherical surface and the second lens group includes at least one aspherical surface.

With constructing in such a manner, it becomes possible to excellently correct various aberrations such as spherical aberration and coma.

A method for focusing a close-up lens capable of focusing an object with an imaging magnification from β=0 through at least β=−0.5 according to the present embodiment, the method comprising steps of: providing the close-up lens including four lens groups; fixing a first lens group and a fourth lens group with respect to an image plane upon focusing; and moving a second lens group and a third lens group along an optical axis upon focusing.

With this method for focusing, it becomes possible to obtain excellent images with an imaging magnification from β=0 to at least β=−0.5.

A method for focusing a close-up lens according to the present embodiment includes steps of: providing the close-up lens including four lens groups; satisfying the following conditional expressions (1) and (2); fixing a first lens group and a fourth lens group with respect to an image plane upon focusing; and moving a second lens group and a third lens group along an optical axis upon focusing: 1.5<β0<2.3  (1) 0.3<β1<0.9  (2) where β0 denotes a lateral magnification of the second lens group upon focusing on infinity, and β1 denotes a lateral magnification of the second lens group upon an imaging magnification β=−1.0.

With this method for focusing, it becomes possible to obtain excellent images with focusing from infinity to a close distance allowing a life-size magnification.

A close-up lens according to each Example of the present embodiment is explained below with reference to accompanying drawings.

EXAMPLE 1

FIG. 1 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 1 upon focusing on infinity.

As shown in FIG. 1, the close-up lens according to Example 1 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object and an aspherical surface on the image side surface, a positive meniscus lens having a convex surface facing the object, and a double convex positive lens.

The second lens group G2 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object and an aspherical surface on the image side surface, and a cemented lens constructed by a double concave negative lens cemented with a double convex positive lens.

The third lens group G3 is composed of, in order from the object, a double convex positive lens, and a cemented lens constructed by a double convex positive lens cemented with a negative meniscus lens having a concave surface facing the object.

The fourth lens group G4 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object, and a cemented lens constructed by a negative meniscus lens having a convex surface facing the object cemented with a positive meniscus lens having a convex surface facing the object.

Various values associated with the close-up lens according to Example 1 are listed in Table 1.

In [Specifications], f denotes a focal length, FNO denotes an f-number, and Bf denotes a back focal length.

In [Lens Data], the first column “N” shows a lens surface number counted in order from the object side, the second column “r” shows a radius of curvature of the lens surface, the third column “d” shows a distance to the next lens surface along the optical axis, the fourth column “vd” shows Abbe number of the medium at d-line (wavelength λ=587.6 nm), and the fifth column “nd” shows refractive index of the medium at d-line (wavelength λ=587.6 nm). In the second column “r”, “r=0.0000” denotes a plane surface. Refractive index of the air nd=1.000000 is omitted.

In [Aspherical Data], a surface number N, aspherical coefficients and conical coefficient are shown when an aspherical surface is exhibited by the following expression:

x = (h²/r)/[1 + {1 − κ(h/r)²}^(1/2)] + C 4 ⋅ h⁴ + C 6 ⋅ h⁶ + C 8 ⋅ h⁸ + C 10 ⋅ h¹⁰ where h denotes a vertical height from the optical axis, x denotes a sag amount which is a distance along the optical axis from the tangent surface at the vertex of the aspherical surface to the aspherical surface at the vertical height h from the optical axis, r denotes a radius of curvature of a reference sphere (paraxial radius of curvature), K denotes a conical coefficient, and C4, C6, C8, C10 denote aspherical coefficients. In [Aspherical Data], “E-n” denotes “×10^(−n)”.

The explanation of reference symbols is the same in the other Examples, so that duplicated explanations are omitted.

TABLE 1 [Specifications] f = 54.9 FNO = 2.88 Bf = 36.33 [Lens Data] N r d νd nd  1) 53.6481 1.2235 37.17 1.834000  2) 23.7149 3.0444  3) 55.1154 2.3000 55.43 1.677900  4) 154.6254 0.0943  5) 50.0781 2.7389 42.71 1.834807  6) −176.5486 D6  7) 106.1229 1.1868 64.03 1.516120  8) 19.4713 4.9300  9) −25.1250 1.7346 36.30 1.620040 10) 37.9551 5.8886 40.77 1.883000 11) −30.3977 D11 12> 0.0000 D12 Aperture Stop S 13) 316.1763 2.9215 82.56 1.497820 14) −42.6285 0.0456 15) 55.1820 4.3365 65.47 1.603000 16) −38.7234 1.0955 23.78 1.846660 17) −139.0482 D17 18) 155.9225 1.0955 25.43 1.805180 19) 28.9155 1.5520 20) 68.4642 1.1868 34.96 1.801000 21) 17.8157 5.6603 23.78 1.846660 22) 126.0536 Bf [Aspherical Data] Surface Number N = 2 K = −5.0082 C4 = 6.42810E−05 C6 = −1.62540E−07 C8 = 6.11660E−10 C10 = −9.13480E−13 Surface Number N = 8 K = 1.9410 C4 = −3.17360E−05 C6 = −1.34580E−07 C8 = 1.79850E−10 C10 = −4.47290E−12 [Variable Distances] β = 0 β = −0.5 β = −1.0 D6 2.50446 6.29642 13.4000 D11 10.38771 6.59575 0.10000 D12 21.88818 11.54539 1.64821 D17 3.51915 13.86323 23.75911 [Values for Conditional Expressions] (1): β0 = 1.998 (2): β1 = 0.63

FIGS. 2A, 2B, 2C are graphs showing various aberrations of the close-up lens according to Example 1, in which FIG. 2A is in a state focusing on infinity, FIG. 2B is in a state where an imaging magnification β=−0.5, and FIG. 2C is in a state where an imaging magnification β=−1.0. In this specification throughout D denotes a lateral magnification of the close-up lens in the entirety thereof.

In respective graphs, FNO denotes an f-number, Y denotes an image height, H denotes an object height, D denotes an aberration curve at d-line (wavelength λ=587.6 nm), G denotes an aberration curve at g-line (wavelength λ=435.6 nm), C denotes an aberration curve at C-line (wavelength λ=656.3 nm), and F denotes an aberration curve at F-line (wavelength λ=486.1 nm). The above-described explanation regarding various aberration graphs is the same as the other Examples.

As is apparent from the respective graphs, the close-up lens according to Example 1 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification β=−1.0.

EXAMPLE 2

FIG. 3 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 2 upon focusing on infinity.

As shown in FIG. 3, a close-up lens according to Example 2 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object and an aspherical surface on the image side surface, a positive meniscus lens having a convex surface facing the object, and a double convex positive lens.

The second lens group G2 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object and an aspherical surface on the image side surface, and a cemented lens constructed by a double concave negative lens cemented with a double convex positive lens.

The third lens group G3 is composed of, in order from the object, a double convex positive lens, and a cemented lens constructed by a double convex positive lens cemented with a negative meniscus lens having a concave surface facing the object.

The fourth lens group G4 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object, and a cemented lens constructed by a negative meniscus lens having a convex surface facing the object cemented with a positive meniscus lens having a convex surface facing the object.

Various values associated with the close-up lens according to Example 2 are listed in Table 2.

TABLE 2 [Specifications] f = 58.0 FNO = 2.88 Bf = 37.45 [Lens Data] N r d νd nd  1) 68.8358 1.3514 39.57 1.804400  2) 25.1596 3.1598  3) 50.5680 3.0890 55.48 1.638540  4) 726.7885 0.0997  5) 61.6542 2.8959 42.71 1.834807  6) −213.3350 D6   7) 114.0007 1.2549 64.03 1.516120  8) 21.4584 5.2000  9) −25.9781 1.8341 36.30 1.620040 10) 45.0791 6.2262 40.77 1.883000 11) −31.6859 D11 12> 0.0000 D12 Aperture Stop S 13) 279.3330 3.0890 82.56 1.497820 14) −45.8650 0.0483 15) 55.7141 4.5852 65.47 1.603000 16) −42.3441 1.1584 23.78 1.846660 17) −171.5862 D17 18) 202.8956 1.1584 25.43 1.805180 19) 30.8234 1.6410 20) 90.5377 1.2549 34.96 1.801000 21) 18.9814 5.9849 23.78 1.846660 22) 242.9593 Bf [Aspherical Data] Surface Number N = 2 K = −5.3148 C4 = 5.58040E−05 C6 = −1.43070E−07 C8 = 5.02630E−10 C10 = −7.75980E−13 Surface Number N = 8 K = 2.1218 C4 = −2.69280E−05 C6 = −9.47080E−08 C8 = 9.70030E−11 C10 = −2.56360E−12 [Variable Distances] β = 0 β = −0.5 β = −1.0 D6 2.62569 6.63506 13.83349 D11 12.29619 8.28682 1.08839 D12 23.27238 12.33659 1.87196 D17 4.49094 15.42810 25.89136 [Values for Conditional Expressions] (1): β0 = 2.00 (2): β1 = 0.64

FIGS. 4A, 4B, 4C are graphs showing various aberrations of the close-up lens according to Example 2, in which FIG. 4A is in a state focusing on infinity, FIG. 4B is in a state where an imaging magnification β=−0.5, and FIG. 4C is in a state where an imaging magnification β=−1.0.

As is apparent from the respective graphs, the close-up lens according to Example 2 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification μ=−1.0.

EXAMPLE 3

FIG. 5 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 3 upon focusing on infinity.

As shown in FIG. 5, a close-up lens according to Example 3 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object and an aspherical surface on the image side surface, a double convex positive lens, and a positive meniscus lens having a convex surface facing the object.

The second lens group G2 is composed of, in order from the object, a double concave negative lens, and a cemented lens constructed by a double concave negative lens cemented with a double convex positive lens.

The third lens group G3 is composed of, in order from the object, a double convex positive lens, and a cemented lens constructed by a double convex positive lens cemented with a negative meniscus lens having a concave surface facing the object.

The fourth lens group G4 is composed of, in order from the object, a cemented lens constructed by a positive meniscus lens having a concave surface facing the object cemented with a double concave negative lens, and a cemented lens constructed by a positive meniscus lens having a concave surface facing the object cemented with a negative meniscus lens having a concave surface facing the object.

Various values associated with the close-up lens according to Example 3 are listed in Table 3.

TABLE 3 [Specifications] f = 60.0 FNO = 2.92 Bf = 37.96 [Lens Data] N r d νd nd  1) 60.3817 1.1000 37.17 1.834000  2) 22.6018 3.0297  3) 3399.2300 2.4000 52.32 1.755000  4) −75.0211 0.1000  5) 28.2032 3.4000 55.53 1.696797  6) 1042.1457 D6   7) −234.0513 1.5000 40.77 1.883000  8) 34.3647 4.2931  9) −29.2081 1.2000 49.82 1.617720 10) 36.5790 7.6000 40.77 1.883000 11) −31.1199 D11 12> 0.0000 D12 Aperture Stop S 13) 91.5369 4.0626 65.47 1.603000 14) −45.3779 0.1018 15) 47.5342 4.9554 65.47 1.603000 16) −31.3415 1.4000 23.78 1.846660 17) −223.9687 D17 18) −4996.8991 2.7000 23.78 1.846660 19) −37.4608 1.4000 43.69 1.720000 20) 28.1629 2.7000 21) −103.6322 3.5000 39.23 1.595510 22) −24.1631 1.3000 40.77 1.883000 23) −60.5172 Bf [Aspherical Data] Surface Number N = 2 K = −2.0292 C4 = 3.42730E−05 C6 = −3.44480E−08 C8 = 1.29790E−10 C10 = −1.21790E−13 [Variable Distances] β = 0 β = −0.5 β = −1.0 D6 2.50000 6.38544 13.10605 D11 13.17882 9.29338 2.57277 D12 16.48970 10.27593 3.94864 D17 2.10000 8.31377 14.64137 [Values for Conditional Expressions] (1): β0 = 3.03 (2): β1 = −0.22

FIGS. 6A, 6B, 6C are graphs showing various aberrations of the close-up lens according to Example 3, in which FIG. 6A is in a state focusing on infinity, FIG. 6B is in a state where an imaging magnification β=−0.5, and FIG. 6C is in a state where an imaging magnification β=−1.0.

As is apparent from the respective graphs, the close-up lens according to Example 3 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification β=−1.0.

EXAMPLE 4

FIG. 7 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 4 upon focusing on infinity.

As shown in FIG. 7, a close-up lens according to Example 4 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object, a double convex positive lens, and a positive meniscus lens having a convex surface facing the object.

The second lens group G2 is composed of, in order from the object, a double concave negative lens having an aspherical surface on the image side surface, and a cemented lens constructed by a negative meniscus lens having a concave surface facing the object cemented with a positive meniscus lens having a concave surface facing the object.

The third lens group G3 is composed of, in order from the object, a double convex positive lens, and a cemented lens constructed by a double convex positive lens cemented with a double concave negative lens.

The fourth lens group G4 is composed of, in order from the object, a cemented lens constructed by a positive meniscus lens having a concave surface facing the object cemented with a double concave negative lens, and a positive meniscus lens having a convex surface facing the object.

Various values associated with the close-up lens according to Example 4 are listed in Table 4.

TABLE 4 [Specifications] f = 64.9 FNO = 2.88 Bf = 37.83 [Lens Data] N r d νd nd  1) 73.1911 1.1897 35.28 1.749497  2) 23.7843 4.6333  3) 89.9396 3.2000 50.74 1.677900  4) −69.3425 0.1081  5) 32.2463 3.4000 60.09 1.640000  6) 119.8334 D6   7) −452.8613 1.2979 44.79 1.744000  8) 42.2438 5.3891  9) −23.9371 2.2712 50.88 1.658440 10) −451.4167 6.3000 40.77 1.883000 11) −27.2977 D11 12> 0.0000 D12 Aperture Stop S 13) 113.4847 3.8657 60.29 1.620410 14) −50.6983 0.1081 15) 53.8201 5.1041 65.47 1.603000 16) −35.6935 1.2978 26.52 1.761820 17) 345.1147 D17 18) −1814.3448 3.2109 27.51 1.755200 19) −42.2202 1.7000 49.32 1.743200 20) 25.3684 0.8470 21) 25.6333 3.4346 56.32 1.568830 22) 55.0236 Bf [Aspherical Data] Surface Number N = 8 K = −0.0421 C4 = −1.60920E−06 C6 = −5.10530E−09 C8 = 1.78050E−11 C10 = −9.31080E−14 [Variable Distances] β = 0 β = −0.5 β = −1.0 D6 2.34559 5.84062 11.23623 D11 14.06385 10.56882 5.17320 D12 21.92885 12.96078 4.11221 D17 1.88582 10.85389 19.70246 [Values for Conditional Expressions] (1): β0 = 3.20 (2): β1 = −0.17

FIGS. 8A, 8B, 8C are graphs showing various aberrations of the close-up lens according to Example 4, in which FIG. 8A is in a state focusing on infinity, FIG. 8B is in a state where an imaging magnification β=−0.5, and FIG. 8C is in a state where an imaging magnification β=−1.0.

As is apparent from the respective graphs, the close-up lens according to Example 4 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification β=−1.0.

EXAMPLE 5

FIG. 9 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 5 upon focusing on infinity.

As shown in FIG. 9, a close-up lens according to Example 5 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object, a positive meniscus lens having a convex surface facing the object, and a double convex positive lens. The most object side negative meniscus lens is an aspherical lens forming an aspherical surface on the image plane I side glass surface by applying a resin layer.

The second lens group G2 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object and an aspherical surface on the image side surface, and a cemented lens constructed by a double concave negative lens cemented with a double convex positive lens.

The third lens group G3 is composed of, in order from the object, a positive meniscus lens having a concave surface facing the object, and a cemented lens constructed by a double convex positive lens cemented with a negative meniscus lens having a concave surface facing the object.

The fourth lens group G4 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object, and a cemented lens constructed by a negative meniscus lens having a convex surface facing the object cemented with a positive meniscus lens having a convex surface facing the object.

Various values associated with the close-up lens according to Example 5 are listed in Table 5.

TABLE 5 [Specifications] f = 50.75 FNO = 2.80 Bf = 35.00 [Lens Data] N r d νd nd  1) 72.3338 1.2394 42.72 1.834810  2) 20.5000 0.2000 38.09 1.553890  3) 21.7574 3.6262  4) 49.4081 2.8000 54.68 1.729157  5) 322.3365 0.0443  6) 43.9944 3.2000 46.63 1.816000  7) −215.5926 D7   8) 89.5564 1.2394 64.03 1.516120  9) 21.0795 5.6430 10) −24.8795 1.6821 36.30 1.620040 11) 53.7747 5.7101 40.77 1.883000 12) −29.6576 D12 13> 0.0000 D13 Aperture Stop S 14) −164.6300 2.5000 82.56 1.497820 15) −43.8195 0.0443 16) 48.5362 5.0000 63.38 1.618000 17) −31.5106 1.0624 23.78 1.846660 18) −87.1486 D18 19) 87.4674 1.1509 25.43 1.805180 20) 27.5622 1.5139 21) 56.1194 1.2837 34.96 1.801000 22) 17.5250 6.0000 23.78 1.846660 23) 76.8622 Bf [Aspherical Data] Surface Number N = 3 K = 1.5218 C4 = −4.94910E−07 C6 = −1.58790E−08 C8 = −6.47580E−11 C10 = −3.02540E−13 Surface Number N = 9 K = −2.3833 C4 = 3.22620E−05 C6 = −1.31840E−07 C8 = 5.38320E−10 C10 = −1.90480E−12 [Variable Distances] β = 0 β = −0.5 β = −1.0 D7 2.73230 6.40936 13.01116 D12 11.62011 7.94305 1.34125 D13 20.51008 10.48069 0.88340 D18 2.96672 12.99737 22.59339 [Values for Conditional Expressions] (1): β0 = 1.79 (2): β1 = 0.74

FIGS. 10A, 10B, 10C are graphs showing various aberrations of the close-up lens according to Example 5, in which FIG. 10A is in a state focusing on infinity, FIG. 10B is in a state where an imaging magnification β=−0.5, and FIG. 10C is in a state where an imaging magnification β=−1.0.

As is apparent from the respective graphs, the close-up lens according to Example 5 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification β=−1.0.

EXAMPLE 6

FIG. 11 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 6 upon focusing on infinity.

As shown in FIG. 11, a close-up lens according to Example 6 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object, a double convex positive lens, a negative meniscus lens having a convex surface facing the object, and a double convex positive lens.

The second lens group G2 is composed of, in order from the object, a double concave negative lens, and a cemented lens constructed by a double concave negative lens cemented with a double convex positive lens.

The third lens group G3 is composed of, in order from the object, a double convex positive lens, and a cemented lens constructed by a double convex positive lens cemented with a negative meniscus lens having a concave surface facing the object.

The fourth lens group G4 is composed of, in order from the object, a cemented lens constructed by a double convex positive lens cemented with a double concave negative lens, a positive meniscus lens having a concave surface facing the object, and a negative meniscus lens having a concave surface facing the object.

Various values associated with the close-up lens according to Example 6 are listed in Table 6.

TABLE 6 [Specifications] f = 59.99 FNO = 2.88 Bf = 35.07 [Lens Data] N r d νd nd  1) 49.6392 2.0000 40.77 1.883000  2) 24.1735 4.5000  3) 107.0994 3.2000 42.72 1.834810  4) −118.9379 0.1000  5) 23.9586 2.2000 70.24 1.487490  6) 22.1799 1.7000  7) 43.4010 3.5000 60.69 1.563840  8) −804.1163 D8   9) −81.7637 1.6000 40.77 1.883000 10) 49.2732 3.4322 11) −35.4331 2.1000 43.69 1.720000 12) 32.3118 8.0000 40.77 1.883000 13) −30.3900 D13 14> 0.0000 D14 Aperture Stop S 15) 69.6837 4.5000 81.61 1.497000 16) −45.6572 0.1000 17) 63.7316 5.3000 70.24 1.487490 18) −33.6688 1.6000 23.78 1.846660 19) −91.3448 D19 20) 661.9601 4.5000 23.78 1.846660 21) −27.3275 1.8000 27.51 1.755200 22) 36.2356 2.6000 23) −215.2877 2.8000 23.78 1.846660 24) −38.5091 0.9000 25) −28.6608 1.5000 52.32 1.755000 26) −121.5032 Bf [Variable Distances] β = 0 β = −0.5 β = −1.0 D8 3.53723 7.45531 13.98947 D13 12.82448 8.90640 2.37224 D14 21.64645 12.66373 3.44637 D19 3.17811 12.16083 21.37819 [Values for Conditional Expressions] (1): β0 = 3.93 (2): β1 = −0.34

FIGS. 12A, 12B, 12C are graphs showing various aberrations of the close-up lens according to Example 6, in which FIG. 12A is in a state focusing on infinity, FIG. 12B is in a state where an imaging magnification β=−0.5, and FIG. 12C is in a state where an imaging magnification β=−1.0.

As is apparent from the respective graphs, the close-up lens according to Example 6 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification β=−1.0.

EXAMPLE 7

FIG. 13 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 7 upon focusing on infinity.

As shown in FIG. 13, a close-up lens according to Example 7 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object and an aspherical surface on the image side surface, a double convex positive lens, and a cemented lens constructed by a negative meniscus lens having a convex surface facing the object cemented with a positive meniscus lens having a convex surface facing the object.

The second lens group G2 is composed of, in order from the object, a double concave negative lens, and a cemented lens constructed by a double concave negative lens cemented with a double convex positive lens.

The third lens group G3 is composed of, in order from the object, a double convex positive lens, and a cemented lens constructed by a double convex positive lens cemented with a negative meniscus lens having a concave surface facing the object.

The fourth lens group G4 is composed of, in order from the object, a positive meniscus lens having a concave surface facing the object, a double concave negative lens, a positive meniscus lens having a concave surface facing the object, and a negative meniscus lens having a concave surface facing the object.

Various values associated with the close-up lens according to Example 7 are listed in Table 7.

TABLE 7 [Specifications] f = 57.6 FNO = 2.88 Bf = 36.95 [Lens Data] N r d νd nd  1) 99.7242 1.3446 37.17 1.834000  2) 25.2586 2.9500  3) 162.4990 3.2654 52.32 1.755000  4) −63.3696 0.0960  5) 37.5818 1.5366 23.78 1.846660  6) 27.4223 3.4574 40.77 1.883000  7) 269.9034 D7   8) −204.9659 1.1525 46.58 1.804000  9) 31.9916 4.4699 10) −26.4063 1.2965 44.89 1.639300 11) 38.5096 7.4431 40.77 1.883000 12) −28.3376 D12 13> 0.0000 D13 Aperture Stop S 14) 88.2222 3.4574 94.97 1.438750 15) −40.8526 0.0978 16) 54.7952 4.5139 81.61 1.497000 17) −29.6922 1.2485 23.78 1.846660 18) −63.2238 D18 19) −59.1213 1.9208 23.78 1.846660 20) −32.4933 0.5282 21) −45.9203 1.3446 41.24 1.701540 22) 41.5215 2.4970 23) −56.7820 2.8812 34.47 1.639800 24) −28.5627 1.2485 25) −18.5941 1.3446 60.09 1.640000 26) −26.8640 Bf [Aspherical Data] Surface Number N = 2 K = 0.5089 C4 = 4.78110E−06 C6 = 1.77850E−09 C8 = 3.09600E−11 C10 = −7.17800E−15 [Variable Distances] β = 0 β = −0.5 β = −1.0 D7 2.26762 5.23755 10.56946 D12 11.02008 8.05013 3.19843 D13 19.08376 11.23975 3.59710 D18 1.91404 9.75805 17.40070 [Values for Conditional Expressions] (1): β0 = 2.59 (2): β1 = −0.20

FIGS. 14A, 14B, 14C are graphs showing various aberrations of the close-up lens according to Example 7, in which FIG. 14A is in a state focusing on infinity, FIG. 14B is in a state where an imaging magnification β=−0.5, and FIG. 14C is in a state where an imaging magnification β=−1.0.

As is apparent from the respective graphs, the close-up lens according to Example 7 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification β=−1.0.

EXAMPLE 8

FIG. 15 is a cross-sectional view showing a lens configuration of a close-up lens according to Example 8 upon focusing on infinity.

As shown in FIG. 15, a close-up lens according to Example 8 is composed of, in order from an object, a first lens group G1 having positive refractive power, a second lens group G2 having negative refractive power, an aperture stop S, a third lens group G3 having positive refractive power, and a fourth lens group G4 having negative refractive power. Upon focusing from infinity to a close distance allowing a life-size magnification, the first lens group G1, the fourth lens group G4, and the aperture stop S are fixed with respect to an image plane I, the second lens group G2 is moved along an optical axis toward the image plane I, and the third lens group G3 is moved along the optical axis toward the object.

The first lens group G1 is composed of, in order from the object, a negative meniscus lens having a convex surface facing the object, a double convex positive lens, and a double convex positive lens.

The second lens group G2 is composed of, in order from the object, a double concave negative lens having an aspherical surface on the image side surface, and a cemented lens constructed by a double concave negative lens cemented with a double convex positive lens.

The third lens group G3 is composed of, in order from the object, a double convex positive lens, and a cemented lens constructed by a double convex positive lens cemented with a double concave negative lens.

The fourth lens group G4 is composed of, in order from the object, a cemented lens constructed by a double convex positive lens cemented with a double concave negative lens, and a positive meniscus lens having a convex surface facing the object.

Various values associated with the close-up lens according to Example 8 are listed in Table 8.

TABLE 8 [Specifications] f = 60.0 FNO = 2.89 Bf = 43.16 [Lens Data] N r d νd nd  1) 61.8055 1.1000 35.28 1.749497  2) 21.7357 4.5000  3) 178.0125 2.8000 50.74 1.677900  4) −81.1807 0.1000  5) 31.8030 3.4000 60.09 1.640000  6) −430.0858 D6   7) −93.9735 1.2001 44.79 1.744000  8) 43.2422 4.7000  9) −26.6717 2.1000 50.88 1.658440 10) 225.7682 5.4000 40.77 1.883000 11) −26.6715 D11 12> 0.0000 D12 Aperture Stop S 13) 76.1926 3.5744 60.29 1.620410 14) −51.5727 0.1000 15) 57.2625 4.7195 65.47 1.603000 16) −31.6111 1.2000 26.52 1.761820 17) 518.3373 D17 18) 1121.7328 2.9689 27.51 1.755200 19) −43.1177 1.8000 49.32 1.743200 20) 23.2742 0.7437 21) 23.6785 3.1758 56.32 1.568830 22) 47.1230 Bf [Aspherical Data] Surface Number N = 8 K = 0.1631 C4 = −2.60650E−06 C6 = −2.93000E−09 C8 = −1.25040E−11 C10 = −3.46030E−14 [Variable Distances] β = 0 β = −0.5 β = −1.0 D6 3.86985 7.10150 12.09051 D11 13.24131 10.00966 5.02065 D12 20.36812 12.07586 3.89411 D17 1.18069 9.47295 17.65470 [Values for Conditional Expressions] (1): β0 = 3.20 (2): β1 = −0.23

FIGS. 16A, 16B, 16C are graphs showing various aberrations of the close-up lens according to Example 8, in which FIG. 16A is in a state focusing on infinity, FIG. 16B is in a state where an imaging magnification β=−0.5, and FIG. 16C is in a state where an imaging magnification β=−1.0.

As is apparent from the respective graphs, the close-up lens according to Example 8 shows superb optical performance as a result of good corrections to various aberrations over entire focusing range from infinity β=0 to a close distance allowing a life-size magnification β=−1.0.

As described above, the present embodiment makes it possible to provide a close-up lens with superb optical performance capable of focusing from infinity to a close distance allowing a life-size magnification by an internal focusing system with small moving amounts of focusing lens groups, an imaging apparatus, and a method for focusing a close-up lens.

Although a close-up lens with a four-lens-group configuration is shown as each Example of the present embodiment, the lens-group configuration according to the present embodiment is not limited to this, other lens-group configurations such as a five-lens-group configuration are possible.

In a close-up lens according to present embodiment, in order to correct an image blur caused by a camera shake, a portion of a lens group, or a single lens group may be moved as a vibration reduction lens group in a direction perpendicular to the optical axis. In a close-up lens according to present embodiment, it is preferable that the fourth lens group or a portion thereof is used for the vibration reduction lens group.

Moreover, any lens surface may be formed as an aspherical surface. The aspherical surface may be fabricated by a fine grinding process, a glass molding process that a glass material is formed into an aspherical shape by a mold, or a compound type process that a resin material is formed into an aspherical shape on a glass lens surface.

An antireflection coating having high transmittance over a broad wavelength range may be applied to each lens surface to reduce flare or ghost images, so that high optical performance with a high contrast can be attained.

FIG. 17 is a diagram showing an imaging apparatus (camera) equipped with a close-up lens according to Example 1.

As shown in FIG. 17, the camera 1 is a single-lens reflex digital camera equipped with the close-up lens according to Example 1 as an image-taking lens 2.

In the camera 1, light emitted from a subject (not shown) is converged by the image-taking lens 2, and focused on a focusing screen 4 through a quick return mirror 3. The subject image focused on the focusing screen 4 is reflected a plurality of times by a pentagonal roof prism 5, and led to an eyepiece 6. Therefore, a photographer can observe the subject image as an erected image through the eyepiece 6.

When the photographer presses a shutter release button (not shown), the quick return mirror 3 is removed from an optical path, and the light from the subject (not shown) reaches an imaging device 7.

Light from the subject is captured by the imaging device 7 and stored in a memory (not shown) as a subject image. In this manner, the photographer can take a picture of the subject by the camera 1.

As shown in Example 1, the close-up lens according to Example 1 of the embodiment attached to the camera 1 as an image-taking lens 2 makes it possible to realize a close-up lens with superb optical performance capable of focusing from infinity to a close distance allowing a life-size magnification by an internal focusing system with small moving amounts of focusing lens groups. Accordingly, the camera 1 makes it possible to realize an imaging apparatus capable of taking a close-up photograph by focusing from infinity to a close distance allowing a life-size magnification by an internal focusing system.

Although the above-described example is shown a case composing a camera 1 equipped with the close-up lens according to Example 1, it is needless to say that the similar effect can be obtained by a camera equipped with a close-up lens according to any one of Examples 2 through 8 of the present embodiment.

The present embodiment only shows a specific example for the purpose of better understanding of the present invention. Accordingly, it is needless to say that the invention in its broader aspect is not limited to the specific details and representative devices shown and described herein, and various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

1. A close-up lens comprising four lens groups, and capable of focusing an object with an imaging magnification from β=0 through at least β=−1.0; wherein upon focusing, a first lens group and a fourth lens group are fixed with respect to an image plane, and a second lens group and a third lens group are moved along an optical axis; and the following conditional expressions being satisfied: 1.5<β0<2.3 0.3<β1<0.9 where β denotes a lateral magnification of the close-up lens in the entirety thereof, β0 denotes a lateral magnification of the second lens group upon focusing on infinity, and β1 denotes a lateral magnification of the second lens group upon an imaging magnification β=−1.0.
 2. The close-up lens according to claim 1, wherein at least one aspherical surface is included in the close-up lens.
 3. The close-up lens according to claim 1, wherein the first lens group is composed of three lenses or less.
 4. The close-up lens according to claim 1, wherein the fourth lens group is composed of at least three lenses or more.
 5. The close-up lens according to claim 1, wherein the first lens group includes at least one aspherical surface.
 6. The close-up lens according to claim 1, wherein the second lens group includes at least one aspherical surface.
 7. An imaging apparatus equipped with the close-up lens according to claim
 1. 8. A method for focusing a close-up lens capable of focusing an object with an imaging magnification from β=0 through at least β=1.0, the method comprising steps of: providing the close-up lens including four lens groups; fixing a first lens group and a fourth lens group with respect to an image plane upon focusing; and moving a second lens group and a third lens group along an optical axis upon focusing; and satisfying the following conditional expressions: 1.5<β0<2.3 0.3<β1<0.9 where β denotes a lateral magnification of the close-up lens in the entirety thereof, β0 denotes a lateral magnification of the second lens group upon focusing on infinity, and 131 denotes a lateral magnification of the second lens group upon the imaging magnification β=−1.0.
 9. The method according to claim 8, further including a step of: including at least one aspherical surface in the close-up lens. 